1. Field of Invention
The present invention relates to a semiconductor manufacturing process. More particularly, the present invention relates to a method of forming a contact opening over a substrate.
2. Description of Related Art
Semiconductor manufacturing can be roughly divided into four modules, including diffusion, etching, thin film and photo-exposure. Photo-exposure is one of the steps in a photolithographic process. After photo-exposure, the pattern on a photomask is transferred to a silicon chip so that the etching module may be conducted to form an etching pattern or to provide a good implant pattern for the film module. Hence, accurate transfer of pattern in photolithographic process is critical to successful fabrication of semiconductor devices.
In photolithographic process, resolution of the exposure station and wavelength xcex of light source is related by the following formula:
R=K1xcex/NA
where R is the resolution of the exposure station, K1 is a constant related to the photoresist material and the processing conditions, and NA is the numeral aperture of the lens system used by the exposure station. According to the formula, resolution R provided by the exposure station is small when wavelength xcex of the light source is short. Similarly, when numerical aperture NA of the exposure station is large, resolution R provided by the exposure station is also small. To transfer a given pattern on a photomask to a photoresist layer accurately, the pattern projected by the exposure station must have a depth of focus (DOF) covering the entire thickness of the photoresist layer. Preferably, identical focusing is obtained at the upper surface of the photoresist layer and the surface close to the substrate. In general, DOF of an exposure station can be quantified using the formula:
DOF=K2xcex/(NA)2
To increase DOF of an exposure station, a light source having long wavelength and a lens system having a small numerical aperture is preferred. However, this presents a direct conflict with the demand for resolution.
Following recent increase in the level of integration for integrated circuits, photomask pattern is getting increasingly dense. Pitches between neighboring bit lines, word lines, doping regions and capacitors are all reduced. Consequently, an exposure station having a higher resolution is required to transfer such fine patterns. Yet, increasing the resolution of an exposure station leads to a loss of depth of focus. On the other hand, if depth of focus of an exposure station is increased to obtain a more accurate pattern transfer, resolution will be lowered. When deep sub-micron devices are manufactured, depth of focus and resolution are optimized by increasing the complexity of photolithographic process and lowering the working life of a photolithographic station. Additional photolithographic steps and planarizing processes are also introduced to reduce the problems encountered during processing, thereby leading to a higher overall production cost.
Accordingly, one object of the present invention is to provide a method of forming a contact opening. First, a substrate is provided. A conductive structure and a dielectric layer are sequentially formed over the substrate. A first photoresist layer is formed over the dielectric layer. The first photoresist layer is exposed to light through a photomask and then the exposed photoresist layer is developed so that opening pattern on the photomask is transferred to the first photoresist layer. Ultimately, a first opening that exposes a portion of the dielectric layer is formed in the first photoresist layer. A second photoresist layer is formed over the patterned first photoresist layer. A relative horizontal shifting between the substrate and the photomask is carried out so that the opening pattern on the photomask is above a portion of the first opening. A second photo-exposure is conducted using the photomask so that the opening pattern is transferred to the second photoresist layer. Consequently, a second opening that exposes a portion of the first photoresist layer and a portion of the first opening is formed in the second photoresist layer. Thereafter, using the first and the second photoresist layer as a mask, a portion of the dielectric layer is removed until a contact opening that exposes a portion of the conductive structure is formed. Finally, the first and the second photoresist layer are removed.
According to one embodiment of this invention, before forming the second photoresist layer, further includes forming a buffer layer over the substrate. The buffer layer includes a hydrophilic anti-reflection coating or a layer having a hydrophilic chemical structure. In addition, there is a positional shift of the second opening relative to the first opening. The positional shift has a magnitude smaller than dimension of the first opening.
Since there is a relative positional shift between the first and the second opening, and furthermore, the second opening exposes a portion of the first photoresist layer and a portion of the first opening, the extent of exposure of the dielectric layer by the second opening is smaller than the first opening. Hence, the photoresist opening exposes a smaller portion of the dielectric layer and size of the ultimately formed contact opening, formed using the first and the second photoresist layer as a mask, is reduced.
Furthermore, the presence of a first photoresist layer and the second photoresist layer is capable of preventing any mixing with the first photoresist layer when the second photoresist layer is formed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.